1. Field of the Invention
The present invention relates to an apparatus for identifying an article using a RFID (Radio Frequency Identification) technique. More particularly, the present invention relates to an apparatus for identifying an article, by attaching a tag having the RFID element to an article, and then interrogating the tag with an interrogator.
2. Discussion of the Background
Conventionally, a tag identifying system includes a single two-way communication channel provided between an interrogator and a plurality of tags within a range of the interrogator. Initially, the identifications of the tags are not known to the interrogator. The tags are activated when the interrogator responds to a signal of a single frequency, and each tag is enabled to inactivate itself (Japanese Unexamined Patent Publication No. 8-316888). In this tag identifying system, during the first broadcast the interrogator broadcasts a first inquiry message including a first signal used for activating all tags within the range of the interrogator and a first value indicating the number of the tags within a range of the interrogator. In addition, a storage unit provided for each tag stores into a memory an identification record not identified by the interrogator by responding to the first inquiry message.
Further, a first residual tag forming unit provided in each tag performs respective calculations based on the first value, random bits for each tag, and the stored identification record in response to a reception of the first inquiry message. The tags are inactivated when the respective calculations result in predetermined consequences, so as to form a first group of activated residual tags. After a first inactivating unit for identifying each tag activated in the first group has modified the identification record stored in the memory of each tag in the first group indicating an identification by the interrogator has succeeded, the first group of tags are inactivated.
Moreover, after the first broadcast, the interrogator broadcasts a second inquiry message including a second signal that activates all tags within the range of the interrogator and a second value indicating the number of the unidentified tags within the range of the interrogator. The second value is used in place of the first value. Further, a second residual tag forming unit and a second inactivating unit performing the same processes as the first residual tag forming unit and the first inactivating unit, and form a second group of activated residual tags so as to increase the number of identified tags. Further, the processes preformed by the second broadcasting unit, the second residual tag forming unit, and the second inactivating unit use the inquiry message having a next signal and a next value until the identification of all tags within the range of the interrogator is complete.
In the above-described tag identifying system, the tags are divided into small groups, and the tags of one group are identified at one time so as to save electrical power by turning off the power supply for the tags not belonging to the group which is presently being identified. Each tag enters the group by performing a calculation with a parameter stored in the tag and a parameter received from the interrogator.
However, in the tag identifying system disclosed in the Japanese Unexamined Patent Publication No. 8-316888, when the tag overlaps with another tag and/or a metal plate, a mutual induction (inducing action) is generated between an antenna coil of the tag and an antenna coil of the other tag or the metal plate. Thus, a resonant frequency of the tag changes. Consequently, the tag no longer oscillates, even if the interrogator forms an alternating magnetic field in the surroundings thereof, i.e., even if a radio wave with which the tag (the tags do not overlap with each other) resonates is transmitted from the transmitting/receiving antenna of the interrogator, and electrical power is no longer supplied to the RFID element of the tag. As a result, a problem occurs because the interrogator can""t identify overlapping tags.
Accordingly, it is an object of the present invention to provide a novel apparatus for identifying an article, in which an interrogator is able to identify a tag, even if the tag overlaps with another tag or a metal plate.
This and other objects are achieved by providing a novel apparatus for identifying an article including a tag attached to the article. The tag has an antenna coil and a RFID element connected to the antenna coil. The RFID element includes a memory for storing data about the article. Also included is an interrogator for activating the tag by transmitting a radio wave having a frequency with which the tag resonates from a transmitting/receiving antenna, and for generating a response signal from the tag by reading data from the memory in response to a reading command, and by writing the data into the memory in response to a writing command. The interrogator includes an oscillation circuit for sweeping a radio wave transmitted from the transmitting/receiving antenna over a predetermined frequency range. The predetermined frequency range is a frequency range which includes all varied resonance frequencies for a plurality of overlapped tags (i.e., the articles having the tags are overlapped). The predetermined frequency range may also be a frequency range which includes all varied resonance frequencies for tags disposed near metal objects, such as coins.
Individually, each of the plurality of tags has the same resonant frequency. However, each tag has a different resonant frequency according to the mutual induction of the antenna coils of the tags between each other. That is, when the plurality of tags are overlapped, each tag has an intrinsic resonant frequency determined by a distance from other overlapped tags or an arrangement thereof.
As a result, according to the present invention, the frequency of an alternating magnetic field formed by the interrogator (the radio wave transmitted by the interrogator) is varied such that all of the overlapped plurality of tags resonate. That is, all of the overlapped plurality of tags can be identified as the oscillation circuit sweeps an oscillation frequency over the frequency range which includes the resonant frequencies of all the superposed plurality of tags from the one with the minimum value to the one with the maximum value.
Further, when at least one tag and at least one metal plate are overlapped, because each tag has an intrinsic resonant frequency determined by a distance from other overlapped tags and metal plates, the frequency of an alternating magnetic field formed by the interrogator (the radio wave transmitted by the interrogator) is varied such that all of the superposed plurality of tags resonate. That is, all of the superposed plurality of tags can be identified, as the oscillation circuit sweeps an oscillation frequency over the frequency range which includes the resonant frequencies of all the superposed tags and metal plates from the frequency with the minimum value to the frequency with the maximum value.